High capacity lightwave transmission cables frequently comprise multiple optical fibers organized in a ribbon or bundled fiber configuration. Conventional bundled fiber cables typically have two or more optical fibers randomly organized at the cable core. In an effort to increase the optical fiber density and space efficiency, optical fiber ribbons were designed. However, optical fiber bundles wherein the optical fibers are randomly organized are still widely used, especially for relatively small fiber counts.
It has long been recognized that bending of optical fibers is a principal signal loss mechanism. The smaller the bend radius (microbend) the more light escapes from the core of the fiber and is lost. When multiple fibers are arrayed in a cable, the microbending problem is influenced by the nature of the array, since bundles of fibers mechanically interact with one another, as well as with the cable sleeve. The use of optical fibers arrayed in ribbons controls that interaction to some degree, but optical fiber ribbons have their own unique microbending behavior. In an optical fiber ribbon with a rectangular cross section, the out-of-plane bending stiffness is significantly lower than the in-plane bending stiffness, giving rise to the so-called preferred bending axis. Among other consequences, this preferential bending characteristic can cause nonrandom stresses on certain fibers in the ribbon during cable loading. These stresses may degrade the signal transmission characteristics of the optical fibers in the cable. Thus optical fiber ribbons present special considerations in cabling.
It is also universally recognized in optical fiber cable design that a preferred approach to controlling microbending losses is to mechanically decouple the optical fibers from the surrounding cable. In this way mechanical impacts and stresses on the cable are not translated, or minimally translated, to the optical fibers. Various techniques have been used to achieve this. Early approaches involved placing the optical fiber or optical fiber bundle loosely in a relatively rigid tube. The object was to allow the fibers to “float” in the tube. In alternative designs, the optical fibers are coated with a primary coating, typically a polymer coating, and a cable sheath applied over the coating, also typically a polymer. The primary coating in this case is made soft, so that stresses experienced by the cable are inefficiently translated to the optical fibers within the cable. In yet another design aimed at the same goal, the optical fibers are coated with a gel to reduce mechanical coupling between the optical fibers and the surrounding cable sheath. See U.S. Pat. No. 6,035,087, issued Mar. 7, 2000.
The term “encasement” as used herein is defined as the primary medium that surrounds the optical fibers.
Optical fiber cabling techniques that have a design goal of decoupling of optical fibers have met with only moderate success. This is partly due to the tendency of the bundled fibers within the cable to buckle or wrinkle when the cable is moderately bent. The wrinkles typically form on the inside radius of the bend. Whereas the bend itself may have a relatively large radius, a radius that is above the range where serious microbending losses would occur, the bends of the wrinkles are much smaller, and easily translate to the optical fibers causing microbending loss. Thus a technique for eliminating or minimizing these wrinkles in bundled optical fiber cables would represent an important advance in the technology.
A particularly thorough discussion of coatings or encasements for optical fiber ribbon cables appears in U.S. Pat. No. 6,317,542 issued Nov. 13, 2001, and in application Ser. No. 10/420,309, filed Apr. 22, 2003.
These references describes a variety of embodiments wherein conformal encasements are used for optical fiber ribbon stacks. The discussion of conformal encasements used to couple optical fibers to the rest of the cable structure is relevant to the discussion below, and these references are incorporated by reference herein.